Serializer/deserializer (SerDes) lanes with lane-by-lane datarate independence

ABSTRACT

A circuit and method enables multiple serializer/deserializer (SerDes) data lanes of a physical layer device (PHY) to operate across a broad range of diversified data rates that are independent from lane to lane. The multiple SerDes data lanes may operate at data rates independent from one another. A single low frequency clock is input to the PHY. A frequency of the single low frequency clock is increased via a common integer-N phase-locked loop (PLL) on the PHY to produce a higher frequency clock. Each of the SerDes data lanes is operated, independently, as a fractional-N PLL that employs the higher frequency clock. Use of the common integer-N PLL enables modulation noise of the fractional-N PLLs to be suppressed by moving the modulation noise to higher frequencies where a level of the modulation noise is filtered, avoiding use of high risk noise cancellation techniques.

BACKGROUND

Physical layer devices (PHYs) may include multipleSerializer/Deserializer (SerDes) lanes. Each SerDes lane may include aserializer block and a deserializer block. The serializer blockgenerally converts data from a parallel format to a serial format. Thedeserializer block generally converts data from a serial format to aparallel format.

SUMMARY

According to an example embodiment, a circuit on a chip for serial dataapplications may comprise a common phase-locked loop (PLL). The commonPLL may be configured to receive an off-chip reference clock signalgenerated external to the chip and produce an on-chip reference clocksignal. The on-chip reference clock signal may be higher in frequencyrelative to the off-chip reference clock signal. The circuit maycomprise a plurality of serializer/deserializer (SerDes) lanes eachcomposed of a respective transmitter, receiver, and fractional-N(frac-N) PLL. The on-chip reference clock signal may be distributed toeach respective frac-N PLL for use in generating a respective outputclock signal with a respective frequency. The respective transmitter andreceiver of each SerDes lane may be configured to operate at respectivetransmit and receive data rates. The respective transmit and receivedata rates may be based on the respective frequency and may beindependent of data rates of other SerDes lanes of the plurality ofSerDes lanes.

The off-chip reference clock signal may be configured to operate in alow frequency range of 100 MHz to 156 MHz and the on-chip referenceclock signal may be configured to operate in a higher frequency range of400 MHz to 600 MHz.

The circuit may be configured to suppress a respective modulation noise,generated by each respective frac-N PLL, by increasing frequency of therespective modulation noise to enable each respective frac-N PLL tofilter the respective modulation noise.

Each respective frac-N PLL of the plurality of SerDes lanes may becomposed of a respective divider and sigma-delta modulator. Eachrespective sigma-delta modulator may be configured to modulate,randomly, a respective divide value of the respective divider.

The common PLL may be an integer PLL with a multiplying factor and themultiplying factor may be configured to suppress an initial portion ofquantization noise introduced by modulating each respective divide valueof each respective divider of each respective frac-N PLL of theplurality of SerDes lanes.

Each respective frac-N PLL of the plurality of SerDes lanes may befurther composed of a respective out-of-band parasitic pole configuredto suppress a remaining portion of the quantization noise. The remainingportion may be at higher offset frequencies relative to offsetfrequencies of the initial portion. The initial portion and theremaining portion may be suppressed below a noise level of noisegenerated from intrinsic phase detector and voltage controlledoscillator (VCO) noise sources on the chip.

The integer PLL may be a first integer PLL and each respectiveout-of-band parasitic pole may be configured high enough to not affectclosed-loop behavior of the respective frac-N PLL. Each respectiveout-of-band parasitic pole in combination with configuration of themultiplying factor may enable jitter and phase noise performance of eachrespective frac-N PLL of the plurality of SerDes lanes to be comparableto that of a second integer PLL that employs a same integer divide valueas the respective frac-N PLL.

Each respective frac-N PLL of the plurality of SerDes lanes may have anorder that is lower relative to that of the respective sigma-deltamodulator.

The order of each respective frac-N PLL may be two and that of therespective sigma-delta modulator may be three.

Each respective frac-N PLL of the plurality of SerDes lanes may becomposed of a respective spread spectrum clock (SSC) pattern generatorconfigured to modulate the respective frac-N PLL, directly, according toa respective SSC profile.

The chip may be a physical layer device (PHY). Each SerDes lane may beconfigured to employ the respective transmit and receive data rates tocommunicate with a respective media access controller (MAC). Therespective transmit and receive data rates may be configured independentof data rates employed by other SerDes lanes of the PHY forcommunicating with other MACs.

According to another example embodiment, a method may comprise producingan on-chip reference clock signal from a common phase-locked loop (PLL)on a chip based on an off-chip reference clock signal generated externalto the chip. The on-chip reference clock signal may be higher infrequency relative to the off-chip reference clock signal. The methodmay comprise distributing the on-chip reference clock signal to eachrespective frac-N PLL of a plurality of serial/deserializer (SerDes)lanes each composed of a respective transmitter, receiver, andfractional-N (frac-N) PLL. The method may comprise generating arespective output clock signal with a respective frequency from eachrespective frac-N PLL based on the on-chip reference clock signaldistributed. The method may comprise operating each respectivetransmitter and receiver of each SerDes lane at respective transmit andreceive data rates based on the respective frequency and may beindependent of data rates of other SerDes lanes of the plurality ofSerDes lanes.

The method may comprise operating the off-chip reference clock signal ina low frequency range of 100 MHz to 156 MHz and operating the on-chipreference clock signal in a higher frequency range of 400 MHz to 600MHz.

The method may comprise suppressing a respective modulation noise,generated by each respective frac-N PLL, by increasing frequency of therespective modulation noise to enable each respective frac-N PLL tofilter the respective modulation noise.

Each respective frac-N PLL of the plurality of SerDes lanes may becomposed of a respective divider and sigma-delta modulator. The methodmay further comprise employing each respective sigma-delta modulator tomodulate, randomly, a respective divide value of the respective divider.

The common PLL may be an integer PLL with a multiplying factor and themethod may further comprise configuring the multiplying factor tosuppress an initial portion of quantization noise introduced bymodulating each respective divide value of each respective divider ofeach respective frac-N PLL of the plurality of SerDes lanes.

Each respective frac-N PLL of the plurality of SerDes lanes may befurther composed of a respective out-of-band parasitic pole and themethod may further comprise configuring each respective out-of-bandparasitic pole to suppress a remaining portion of the quantizationnoise. The remaining portion may be at higher offset frequenciesrelative to offset frequencies of the initial portion. The initialportion and the remaining portion may be suppressed below a noise levelof noise generated from intrinsic phase detector and voltage controlledoscillator (VCO) noise sources on the chip.

The integer PLL may be a first integer PLL and configuring eachrespective out-of-band parasitic pole may include configuring eachrespective out-of-band parasitic pole to be high enough to not affectclosed-loop behavior of the respective frac-N PLL. Each respectiveout-of-band parasitic pole in combination with configuration of themultiplying factor may enable jitter and phase noise performance of eachrespective frac-N PLL of the plurality of SerDes lanes to be comparableto that of a second integer PLL that employs a same integer divide valueas the respective frac-N PLL.

The method may further comprise configuring each respective frac-N PLLof the plurality of SerDes lanes to have an order that is lower relativeto that of the respective sigma-delta modulator. The order of eachrespective frac-N PLL may be two and that of the respective sigma-deltamodulator may be three.

Each respective frac-N PLL of the plurality of SerDes lanes may becomposed of a respective spread spectrum clock (SSC) pattern generatorand the method may further comprise configuring each respective SSCpattern generator to modulate the respective frac-N PLL, directly,according to a respective SSC profile.

The chip may be a physical layer device (PHY), and the method mayfurther comprise configuring each SerDes lane to employ the respectivetransmit and receive data rates to communicate with a respective mediaaccess controller (MAC) and configuring the respective transmit andreceive data rates independent of data rates employed by other SerDeslanes of the PHY for communicating with other MACs.

It should be understood that example embodiments disclosed herein can beimplemented in the form of a method, apparatus, system, or computerreadable medium with program codes embodied thereon.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing will be apparent from the following more particulardescription of example embodiments, as illustrated in the accompanyingdrawings in which like reference characters refer to the same partsthroughout the different views. The drawings are not necessarily toscale, emphasis instead being placed upon illustrating embodiments.

FIG. 1 is a block diagram of an example embodiment of a circuit on achip for serial data applications.

FIG. 2 is a block diagram of an example embodiment of an apparatus forserial data applications.

FIG. 3 is a block diagram of an example embodiment of a circuit forserial data applications.

FIG. 4 is a block diagram of another example embodiment of a circuit forserial data applications.

FIG. 5 is a circuit diagram of an example embodiment of a prior artphase locked loop (PLL).

FIG. 6 is a graph of an example embodiment of phase noise of the priorart PLL of FIG. 5.

FIG. 7 is a circuit diagram of an example embodiment of a fractional-N(frac-N) PLL.

FIG. 8 is a graph of an example embodiment of phase noise of the frac-NPLL of FIG. 7 with a second order sigma-delta modulator.

FIG. 9 is a graph of an example embodiment of an improved phase noiseresponse relative to the phase noise response of FIG. 8.

FIG. 10 is a graph of another improved phase noise response relative tothe phase noise response of FIG. 8.

FIG. 11 is a graph of a phase noise response for an example embodimentof a synthesizer in which an input frequency is increased to 500 MHz.

FIG. 12 is a graph of an example embodiment of a phase response for afrac-N PLL that employs a high frequency, out-of-band pole.

FIG. 13 is a graph 1300 of a phase response of an example embodiment ofa synthesizer with a modulator that is dithered at a low frequency sothat it is sufficiently randomized.

FIG. 14 is a circuit diagram of an example embodiment of another frac-NPLL.

FIG. 15 is a graph of an example embodiment of a simulated PLL output.

FIG. 16A is flow diagram of an example embodiment of a method forachieving serial/deserializer (SerDes) lane datarate independence.

FIG. 16B is flow diagram of another example embodiment of a method forachieving SerDes lane datarate independence.

DETAILED DESCRIPTION

A description of example embodiments follows.

In serial data applications it may be useful to haveserializer/deserializer (SerDes) lanes (also referred to interchangeablyherein as SerDes data lanes) of a physical layer device (PHY) operateover a broad range of data rates and serial protocols. In addition, formaximum flexibility in connecting to external components, it may also beuseful to have the PHY connect to multiple logical layer media accesscontrollers (MACs) so that the PHY can be programmed at chip startup tocommunicate with a particular protocol for a particular application orboard design while using a same piece of provider silicon. As such, aPHY with SerDes lanes configured to span a wide range of operationalfrequencies and protocol requirements may be useful.

Within the PHY, also referred to interchangeably herein as a SerDes PHY,a plurality of SerDes lanes may be configured to communicate withexternal devices. The SerDes lanes may be composed primarily of atransmitter (TX) and a receiver (RX) to enable such communication.Embodiments disclosed herein enable multiple SerDes data lanes of a PHYto operate across a broad range of diversified data rates from lane tolane without placing difficult constraints with respect to clockingrequirements for clocking signal being provided to the SerDes datalanes. An example embodiment relieves a customer who is employing thePHY in their design from adhering to such difficult constraints. Forexample, the customer may employ low frequency clock signals readilyavailable in their design and are relieved from having to generate ahigh frequency clock signal that may otherwise be required by the PHY.

According to an example embodiment, each SerDes lane may operate at acompletely independent data rate from one another based on a single lowfrequency reference clock signal input. Achieving such lane-by-lanedatarate independence may be enabled by operating each SerDes lane as afractional-N (frac-N) frequency synthesizer and by using a common phaselocked loop (PLL) on the PHY as a frequency step-up PLL for increasingfrequency of the single low frequency reference clock signal input.

The use of the step-up PLL avoids having to do any high riskquantization noise cancellation techniques that exist in the art andprovides an additional benefit of suppressing a frac-N modulation noiseby moving the frac-N noise to higher frequencies where such noise may befiltered by the frac-N PLL. Example embodiments of a fractionalmodulator, PLL loop dynamics, and additional PLL filtering poles enablesa level of the frac-N modulation noise to be suppressed to a point wherephase noise and jitter of the frac-N PLL appears identical to that of aninteger-N PLL, as disclosed further below.

FIG. 1 is a block diagram 100 of an example embodiment of a circuit 102on a chip 104 for serial data applications. The circuit 102 comprises acommon phase-locked loop (PLL) 106. The common PLL 106 is configured toreceive an off-chip reference clock signal 108 generated external to thechip 104 and to produce an on-chip reference clock signal 110. Theon-chip reference clock signal 110 is higher in frequency relative tothe off-chip reference clock signal 108. The circuit 102 comprises aplurality of serializer/deserializer (SerDes) lanes, that is, the SerDeslanes 112 a-n that may be any suitable number of SerDes lanes. Each ofthe SerDes lanes 112 a-n is composed of a respective transmitter,receiver, and fractional-N (frac-N) PLL, such as the transmitters 114a-n, receivers 116 a-n, and frac-N PLLs 118 a-n of the SerDes lanes 112a-n, respectively. The on-chip reference clock signal 110 is distributedto each respective frac-N PLL, that is, the frac-N PLLs 118 a-n, for usein generating a respective output clock signal, that is, the outputclock signals 120 a-n of the frac-N PLLs 118 a-n, respectively, that areeach associated with a respective frequency. The respective transmitterand receiver of each SerDes lane, such as the transmitters 114 a-n andreceivers 118 a-n of the SerDes lanes 112 a-n, respectively, may each beconfigured to operate at respective transmit and receive data rates. Therespective transmit and receive data rates may be based on therespective frequency and may be independent of data rates of otherSerDes lanes of the plurality of SerDes lanes that include the SerDeslanes 112 a-n. The transmit and receive data rates may be a same datarate. Alternatively, the transmit and receive data rates may bedifferent data rates.

The off-chip reference clock signal 108 may be configured to operate ina low frequency range of 100 MHz to 156 MHz and the on-chip referenceclock signal 110 may be configured to operate in a higher frequencyrange of 400 MHz to 600 MHz.

The chip 104 may be a physical layer device (PHY). Each SerDes lane 112a-n may be configured to employ the respective transmit and receive datarates to communicate with a respective media access controller (MAC)(not shown) and external devices (not shown). The respective transmitand receive data rates may be configured independent of data ratesemployed by other SerDes lanes of the PHY for communicating with otherMACs.

FIG. 2 is a block diagram 200 of an example embodiment of an apparatus201 for serial data communications. The apparatus 201 includes a PHY 204that is composed of a circuit (not shown), such as the circuit 102disclosed above with reference to FIG. 1, that enables SerDes lanes ofthe PHY 204 to operate at independent data rates on a lane-by lanebasis.

The PHY 204 is configured to communicate with an outside world 203, suchas an external network or external device, via a first interface 205.The first interface 205 may be an interface to a physical medium, suchas an optical fiber or copper cable, or may be a wireless interface orany other suitable interface for communicating with external devices.

The apparatus 201 may comprise multiple logical layer media accesscontrollers (MACs), such as the MACs 207 a-n, and a second interface209. The second interface 209 may be a logical multiplexer or mediaindependent interface (MII) disposed between the PHY 204 with theplurality of MACs 207 a-n. The plurality of MACs 207 a-n may haverespective requirements for communication, such as respectiverequirements for data rates. Each MAC of the plurality of MACs 207 a-nmay be configured to deliver data between an internal bus 211 of theapparatus 201 and the outside world 203 via a respective SerDes lane(not shown) of the PHY 204. According to an example embodiment, suchrespective SerDes lanes may be configured to operate at respectivetransmit and receive data rates that are independent of data ratesemployed by other SerDes lanes of the PHY 204.

According to an example embodiment of the circuit (not shown) of the PHY204, such as the example embodiment of the circuit 102 of FIG. 1,disclosed above, each SerDes lane (not shown) of the PHY 204 may beoperated independently and need not be grouped with any other SerDeslane based on a common interface requirement for communicating with itsrespective MAC of the plurality of MACs 207 a-n.

FIG. 3 is a block diagram 300 of an example embodiment of a circuit 302for serial data applications. The circuit 302 includes four SerDeslanes, that is, the SerDes lanes 312 a-d, along with a common (COM)block 313. It should be understood that the four SerDes lanes, alsoreferred to herein as a quad lane module (QLM) configuration, is forillustrative purposes and that any suitable number of SerDes lanes maybe employed. Each SerDes lane includes a TX, RX, and frac-N PLL. Forexample, the SerDes lane 312 a includes the TX 314 a, RX 318 a, andfrac-N 316 a. The SerDes lane 312 b includes the TX 314 b, RX 318 b, andfrac-N 316 b. The SerDes lane 312 c includes the TX 314 c, RX 318 c, andfrac-N 316 c, while the SerDes lane 312 d includes the TX 314 d, RX 318d, and frac-N 316 d.

Each SerDes lane further includes digital circuitry to interface to arespective logical MAC (not shown) that may be placed by place and route(PNR) tools and referred to interchangeably herein as PNR. For example,the SerDes lanes 312 a, 312 b, 312 c, and 312 d include the PNR 322 a,322 b, 322 c, and 322 d, respectively, for interfacing to respectivelogical MACs (not shown). Included in the COM block 313 is controlcircuitry that includes the PNR control circuitry 323 that may be usedfor controlling the PNR 322 a, 322 b, 322 c, and 322 d. The COM block313 further includes a common PLL 306 that may be an integer PLL and COMcircuitry 324 that may include miscellaneous circuitry that is commonwithin the circuit 302.

FIG. 4 is a block diagram 400 of another example embodiment of a circuit402 for serial data applications. The circuit 402 includes two SerDeslanes, that is, the SerDes lanes 412 a and 412 b, along with a COM block413. It should be understood that the two SerDes lanes, also referred toherein as a dual lane module (DLM) configuration, is for illustrativepurposes and that any suitable number of SerDes lanes in combinationwith the COM block 413 may be employed based on practical connectivityto MACs and any distribution constraints of signals from the COM block413 to the individual SerDes lanes of the circuit 402.

Each SerDes lane includes a TX, RX, and frac-N PLL. For example, theSerDes lane 412 a includes the TX 414 a, RX 418 a, and frac-N 416 a,while the SerDes lane 412 b includes the TX 414 b, RX 418 b, and frac-N416 b. Each SerDes lane, that is, the SerDes lane 412 a and the SerDeslane 412 b, further includes digital circuitry to interface to arespective logical MAC (not shown), that is, the PNR 422 a and PNR 422b, respectively. Included in the COM block 413 is control circuitry thatmay include the PNR control 423 that may be used for controlling the PNR422 a and PNR 422 b. The COM block 413 further includes a common PLL 406that may be an integer PLL as well as the COM circuitry 424 that mayinclude miscellaneous circuitry that is common within the circuit 402.

FIG. 5 is a circuit diagram 500 of an example embodiment of a prior artphase locked loop (PLL) 530. The PLL 530 may be used by a SerDes TX (notshown) to clock outgoing data and a SerDes RX (not shown) to clockincoming data. The PLL 530 includes a number of elements, including (butnot limited to) a voltage controlled oscillator (VCO) 532, feedbackdivider 534, phase frequency detector (PFD) 536, and loop filter (LF)538. The PLL 530 takes in an input reference clock (REF) 540 andcompares it to a phase of the divided down VCO output 542 at the PFD536. An error signal 544 at the output of the PFD 536 is filtered by theLF 538 to produce a filtered output 546 that may be used to control theVCO 532 that generates an output signal 548 with an output frequencythat may depend on a control voltage of the filtered output 546.

The LF 538 may be any suitable type of loop filter, such as anintegrating loop filter, and the PFD 536 may include a charge-pump (notshown). The output frequency of the output signal 548 may be configuredto be F_(out)=N*F_(ref), where N is an integer value, such as theinteger N 550 that may be combined and input to the divider 534. Assuch, the PLL 530 may be referred to as an integer PLL. A reset block552 may be employed to reset the PLL 530.

Alternatively, a pre-scaler R (not shown) may be configured to dividedown the reference clock 540 and a post-scaler P (not shown) may beconfigured to divide down the output frequency of the output signal 548.As such, the output frequency of the output signal 548 may instead beF_(out)=N*F_(ref)/(R*P) so that some fractional values may be obtained.A stability of an output signal generated by such a PLL, referred to asa fractional PLL, is closely related to a ratio of (F_(ref)/R)/(PLLbandwidth). The PLL bandwidth is often set by other considerations forhigh performance applications, requiring that R=1. The post-scaler P(not shown) is often set to fixed ratios because the VCO 532 may beoperating near a highest rate possible to generate clocks needed toachieve particular data rates. The post-scaler P (not shown) may be usedto generate output phases. As such, a frequency resolution of the outputsignal 548 for data rates in practical applications is generally set byN, which is an integer.

FIG. 6 is a graph 600 of an example embodiment of phase noise of theprior art PLL 530 (also referred to interchangeably herein as asynthesizer) of FIG. 5. In the graph 600, phase noise 658 with respectto frequency offset 660 is plotted. The graph 600 has plots for thephase noise including plots of the detector phase noise, VCO phasenoise, and total phase noise, that is the detector noise 652, VCO noise650, and total noise 654, respectively, for the PLL 530. Such phasenoise with respect to frequency offset is a typical performance metricfor a PLL.

In the graph 600, the VCO noise 650 is the VCO phase noise of the VCO532 and the detector noise 652 is the phase noise of the PFD 536. LFnoise (not shown) of the LF 538 also contributes noise, but has beenwithheld from the graph 600 so as not to confuse the graph 600. In theexample embodiment, it can be assumed that the LF noise (not shown) isset low enough to not be dominant in the PLL 530. A total PLL noise 654for the PLL 530 corresponds to a sum of the two noise sources, that is,the VCO noise 650 and the PFD noise 652, with respect to the frequencyoffset 660.

The PFD noise 652 is low pass filtered by the PLL 530, while the VCOnoise 650 is high pass filtered. As such, the PFD noise 652 is dominantat low frequencies and the VCO noise 650 is dominant at highfrequencies. An integration of the total PLL noise 654 may be convertedto total jitter, which, in the example embodiment, is 372 fsroot-mean-square (rms). In the example embodiment of the PLL 530, afrequency of the input reference clock signal 540 is assumed to be in arange of 100-156 MHz, which is typical for modern SerDes applications.In order to de-couple a choice of frequency resolution (and thereforedata rate resolution) from the reference clock 540, an exampleembodiment may convert the PLL 530 into a fractional-N synthesizer (alsoreferred to interchangeably herein as a PLL) as disclosed with regard toFIG. 7, below.

FIG. 7 is a circuit diagram 700 of an example embodiment of afractional-N (frac-N) PLL 716. A reset block 752 may be employed toreset the PLL 716. The frac-N PLL 716 includes a number of elements,including (but not limited to) a voltage controlled oscillator (VCO)732, feedback divider 734, phase frequency detector (PFD) 736, and loopfilter (LF) 738. The frac-N PLL 716 takes in an input reference clocksignal (REF) 740 and compares it to a phase of the divided down VCOoutput 742 at the PFD 736. An error signal 744 at the output of the PFD736 is filtered by the LF 738 to produce a filtered output 746 that maybe used to control the VCO 732 that generates an output signal 748 withan output frequency that may depend on a control voltage of the filteredoutput 746. The LF 738 may be any suitable type of loop filter, such asan integrating loop filter, and the PFD 736 may include a charge-pump(not shown).

In the example embodiment of the PLL 716, a divide value N 750 ismodulated between integer values in a way that, on average, a fractionaldivide value N·F 761 is obtained where N is an integer and F is afractional value. Modulation of the divide value N 750 may introducequantization noise. An example embodiment may modulate the divider 734of the PLL 716 in randomized manner using a sigma-delta modulator 760 toshape the quantization noise so that the quantization noise is high-passfiltered and then low-pass filtered by the PLL 716. The input referenceclock signal (REF) 740 may drive the sigma-delta modulator 760 in anysuitable manner. For example, the input reference clock signal (REF) 740may drive the sigma-delta modulator 760, directly, or in a buffered form762. Further, the input reference clock signal (REF) 740 may be invertedor altered in some other suitable manner to meet timing margins for thePLL 716.

FIG. 8 is a graph 800 of an example embodiment of phase noise of thefrac-N PLL of FIG. 7 with a second order sigma-delta modulator employedas the sigma-delta modulator 760. In the graph 800, phase noise 858 withrespect to frequency offset 860 is plotted. Phase noise of thesigma-delta modulator 760, that is, the sigma-delta (S-D) noise 851, isplotted along with the detector noise 852 and the VCO noise 850, of thePFD 736 and the VCO 732, respectively, as well as the total noise 854.In the example embodiment, the PLL output frequency of the PLL outputsignal 748 is F_(out)=N·F*F_(ref), where N is an integer and F is afractional value.

According to an example embodiment, the phase noise response of FIG. 8may be improved, as a major hump 855 in the total phase noise 854 is dueto the frac-N modulation process. Specifically, integrated jitter hasincreased from 372 fs rms to 2.294 ps rms. Noise due to the frac-Nmodulation process is denoted as the S-D noise 851 and is large becausemodulation of the divider 734 can only occur at F_(ref), that is, aninput frequency of the input reference clock signal 740 which is only at100 to 156 MHz, that is, a frequency range that customers are typicallywilling to provide. A magnitude of such quantization phase noise such asthe major hump 855, may be derived from the equation:

$\begin{matrix}{\frac{\Delta^{2}}{12} \cdot \left\lbrack {2\mspace{11mu}{\sin\left( \frac{\pi\; f}{f_{ref}} \right)}} \right\rbrack^{2\; M}} & (1)\end{matrix}$

In Eq. (1), above, A represents a quantization step size (1 divide valuestep), M is an order of the sigma-delta modulator, where (M=2 in theexample embodiment of FIG. 8) and f_(ref) is a reference frequency, thatis, the reference frequency of the input reference clock signal 740,that is input to the PLL. Based on Eq. (1), above, options for reducingan impact of the S-D noise 751 may include reducing Δ, increasingf_(ref); adjusting M, or performing a combination thereof.

FIG. 9 is a graph 900 of an example embodiment of an improved phasenoise response relative to the phase noise response of FIG. 8, disclosedabove. In the graph 900, phase noise 958 with respect to frequencyoffset 960 is plotted. Phase noise of the sigma-delta modulator 760,that is, the sigma-delta (S-D) noise 951, is plotted along with thedetector noise 952 and the VCO noise 950, of the PFD 736 and the VCO732, respectively, as well as the total noise 954. To achieve theimproved phase noise response, a magnitude of the quantization stepsize, that is, Δ, is reduced. To reduce the magnitude of thequantization step size, the quantization noise may be partiallycancelled or sub-phases of the VCO 732 may be used as a step size inperforming the feedback selection and modulation comparison (so thatΔ<1). However, either of these options are costly in area and power andrisky in that they don't offer guaranteed reduction of the quantizationnoise because they rely on matching of phases or cancellation paths. Anexample embodiment disclosed herein takes advantage of the design of Eq.(1), disclosed above, to virtually remove an impact of the quantizationnoise without using any complex noise cancellation methods.

FIG. 10 is a graph 1000 of another improved phase noise responserelative to the phase noise response of FIG. 8, disclosed above. In thegraph 1000, phase noise 1058 with respect to frequency offset 1060 isplotted. Phase noise of the sigma-delta modulator 760, that is, thesigma-delta (S-D) noise 1051, is plotted along with the detector noise1052 and the VCO noise 1050, of the PFD 736 and the VCO 732,respectively, as well as the total noise 1054. In the example embodimentof FIG. 10, a 3^(rd) order modulator is employed as the sigma-deltamodulator 760. In the example embodiment of FIG. 10, the quantizationnoise has been moved to higher frequencies, in accordance with Eq. (1),disclosed above. While the total rms jitter is now reduced to 1.885 ps,the quantization noise may still be considered unacceptably high.

FIG. 11 is a graph 1100 of a phase noise response for an exampleembodiment of a synthesizer in which an input frequency is increased to500 MHz. The graph 1100 shows an improved phase noise response relativeto the phase noise response of FIG. 10, disclosed above. In the graph1100, phase noise 1158 with respect to frequency offset 1160 is plotted.Phase noise of the sigma-delta modulator 760, that is, the sigma-delta(S-D) noise 1151, is plotted along with the detector noise 1152 and theVCO noise 1150, of the PFD 736 and the VCO 732, respectively, as well asthe total noise 1154. In the example embodiment, the reference frequencyof the input reference clock signal 740, that is, the input frequencyF_(ref), is increased to 500 MHz.

In the example embodiment, the quantization noise is reduced by shiftingit to higher frequency, where a loop filter, such as the LF 738 of thePLL 716 of FIG. 7, disclosed above, may be more effective in filteringit. The rms jitter of the example embodiment of FIG. 11 is reduced to381 fs. The higher F_(ref) may be generated by an on-chip PLL, such asthe common PLL 106, the common PLL 306, or the common PLL 406 of FIG. 1,FIG. 3, or FIG. 4, respectively. The on-chip PLL may be configured tofrequency multiply an off-chip low frequency reference clock signal witha frequency in a range from 100-156 MHz to another frequency, alsoreferred to herein as a sigma delta reference, that is in a range of400-600 MHz.

An exact frequency of such a sigma delta reference is not important.Such a frequency may be configured such that it is “high enough” to getsufficient suppression of the quantization noise. Such a frequency maybe in the 400-600 MHz range, a frequency that may be distributed easilyacross a large die for all SerDes lanes from a common multiplier PLL,such as the common PLL 106, the common PLL 306, or the common PLL 406 ofFIG. 1, FIG. 3, or FIG. 4, respectively. An additional benefit of suchan embodiment is that by moving a SerDes lane PLL reference clock to ahigher frequency, reference clock spur suppression may be more effectiveas the SerDes lane PLL reference clock may now be configured with whatis considered to be a high frequency.

FIG. 12 is a graph 1200 of an example embodiment of a phase response fora frac-N PLL that employs a high frequency, out-of-band pole. Phasenoise of the sigma-delta modulator 760, that is, the sigma-delta (S-D)noise 1251, is plotted along with the detector noise 1252 and the VCOnoise 1250, of the PFD 736 and the VCO 732, respectively, as well as thetotal noise 1254. According to the example embodiment, the pole may beconfigured to have minimal impact of closed loop synthesizer performancearound the PLL bandwidth, that is, it has no impact with regard to thePLL bandwidth or peaking, yet offers additional filtering suppression athigher offset frequencies. As such, the out-of-band pole suppresses theremaining quantization noise without adding additional power.

In the example embodiment of FIG. 12, the rms jitter is increasedslightly to 390 fs rms, which is the result of the closed loop polesmoving slightly due to the presence of a parasitic pole at 30 MHz. Thisis a negligible performance delta and could be further optimized. Anadditional benefit of such a parasitic pole is that it will filter anyreference spur present at 500 MHz substantially more than the PLL wouldotherwise. More importantly, a performance of such a frac-N synthesizerin terms of jitter and phase noise is now comparable to that of aninteger-N PLL. It should be understood that the frac-N synthesizeremploys a fractional divide value of N·F, where N is an integer and F isa fractional value, and that the performance of such a frac-Nsynthesizer (i.e., frac-N PLL) is comparable in terms of jitter andphase noise to that of the integer-N PLL, where the integer-N PLLemploys a same integer divide value for N as the frac-N synthesizer.

The common PLL 106 may be an integer PLL with a multiplying factor andthe multiplying factor may be configured to suppress an initial portionof quantization noise introduced by modulating each respective dividevalue of each respective divider of each respective frac-N PLL 116 a-nof the plurality of SerDes lanes 112 a-n. Each respective frac-N PLL 116a-n of the plurality of SerDes lanes 112 a-n may be further composed ofa respective out-of-band parasitic pole (not shown) that is configuredto suppress a remaining portion of the quantization noise. The remainingportion may be at higher offset frequencies relative to offsetfrequencies of the initial portion. The initial portion and theremaining portion may be suppressed below a noise level of noisegenerated from intrinsic phase detector (not shown) and voltagecontrolled oscillator (VCO) noise sources (not shown) on the chip 104.

Each respective out-of-band parasitic pole (not shown) may be configuredhigh enough to not affect closed-loop behavior of the respective frac-NPLL 116 a-n. Each respective out-of-band parasitic pole (not shown) incombination with configuration of the multiplying factor may enablejitter and phase noise performance of each respective frac-N PLL of theplurality of SerDes lanes to be comparable to that of the an integer PLLthat employs a same integer divide value as the respective frac-N PLL,such as disclosed above with regard to FIG. 12.

FIG. 13 is a graph 1300 of a phase response of an example embodiment ofa synthesizer with a modulator that is dithered at a low frequency sothat it is sufficiently randomized. Phase noise of the sigma-deltamodulator 760, that is, the sigma-delta (S-D) noise 1351, is plottedalong with the detector noise 1352 and the VCO noise 1350, of the PFD736 and the VCO 732, respectively, as well as the total noise 1354. Suchdithering avoids fractional spurs from appearing in the spectrum, whichwould translate into additional jitter. According to an exampleembodiment, such dithering may be performed by coupling a leastsignificant bit (LSB) of the modulator, such as the sigma-deltamodulator 760, to a high reference value, or by adding a pseudorandombinary sequence (PRBS) of sufficient length to the LSB of the modulator.

FIG. 14 is a circuit diagram 1400 of an example embodiment of anotherfrac-N PLL 1416. A reset block 1452 may be employed to reset the PLL1416. The frac-N PLL 1416 includes a number of elements, including (butnot limited to) a voltage controlled oscillator (VCO) 1432, feedbackdivider 1434, phase frequency detector (PFD) 1436, and loop filter (LF)1438. The frac-N PLL 1416 takes in an input reference clock signal (REF)1440 and compares it to a phase of the divided down VCO output 1442 atthe PFD 1436. An error signal 1444 at the output of the PFD 1436 isfiltered by the LF 1438 to produce a filtered output 1446 that may beused to control the VCO 1432 that generates an output signal 1448 withan output frequency that may depend on a control voltage of the filteredoutput 1446. The LF 1438 may be any suitable type of loop filter, suchas an integrating loop filter, and the PFD 1436 may include acharge-pump (not shown). The frac-N PLL 1416 further includes asigma-delta modulator 1460 and a digital spread-spectrum profilegenerator, that is, the spread spectrum clock (SSC) pattern generator1461 that may be configured to modulate the synthesizer.

Such modulation is useful because many serial data standards requiregeneration of spread spectrum clocks (SSC). Without frac-N capabilities,it becomes complex to generate SSC profiles and such complex generationmay tap multiple VCO output phases or be based on generation of multipleoutput phases to create such profiles. According to the exampleembodiment of FIG. 14, the frac-N PLL 1416 may be modulated, directly,obviating such complex generation.

Turning back to FIG. 1, each respective frac-N PLL 116 a-n of theplurality of SerDes lanes 112 a-n may be composed of a respectivedivider (not shown) and sigma-delta modulator (not shown), such as thedivider 1434 and sigma-delta modulator 1460 of FIG. 14, disclosed above.Each respective sigma-delta modulator may be configured to modulate,randomly, a respective divide value of the respective divider. Eachrespective frac-N PLL 116 a-n of the plurality of SerDes lanes 112 a-nmay have an order that is lower relative to that of the respectivesigma-delta modulator. The order of each respective frac-N PLL may betwo and that of the respective sigma-delta modulator may be three;however, any suitable order may be employed for each respective frac-NPLL and the respective sigma-delta modulator. For example, for practicalreasons an order of the respective sigma-delta modulator may be greaterthan or equal to one and an order of each respective frac-N PLL may beof order two or higher. Orders of each respective frac-N PLL andsigma-delta modulator may be co-optimized to yield best phase noise andjitter performance in the circuit 102. The circuit 102 may be configuredto suppress a respective modulation noise, generated by each respectivefrac-N PLL 116 a-n, by increasing frequency of the respective modulationnoise to enable each respective frac-N PLL to filter the respectivemodulation noise.

FIG. 15 is a graph 1500 of an example embodiment of a simulated PLLoutput 1548. The graph 1500 plots the frequency 1590 over time 1529 forthe simulated PLL output 1548. The simulated PLL output 1548 is asimulated output signal of a frac-N PLL, such as the frac-N PLL 1416 ofFIG. 14, disclosed above, modulated by a down-spread SSC profile, suchas the frac-N PLL of FIG. 14, disclosed above, according to the SerialATA (Advanced Technology Attachment) (SATA) specification. According tothe example embodiment, a triangular down-spreading profile is achieved.It should be understood that other suitable profiles, such as up spread,center spread, square-wave, etc., may be employed. Turning back to FIG.1, each respective frac-N PLL 116 a-n of the plurality of SerDes lanes112 a-n may be composed of a respective spread spectrum clock (SSC)pattern generator, such as the SSC pattern generator 1461 of FIG. 14,disclosed above, that may be configured to modulate the respectivefrac-N PLL, directly, according to a respective SSC profile (not shown).

FIG. 16A is flow diagram 1600 of an example embodiment of a method forachieving serial/deserializer (SerDes) lane datarate independence. Themethod may begin (1602) and produce an on-chip reference clock signalfrom a common phase-locked loop (PLL) on a chip based on an off-chipreference clock signal generated external to the chip (1604). Theon-chip reference clock signal may be higher in frequency relative tothe off-chip reference clock signal. The method may distribute theon-chip reference clock signal to each respective frac-N PLL of aplurality of serial/deserializer (SerDes) lanes each composed of arespective transmitter, receiver, and fractional-N (frac-N) PLL (1606).The method may generate a respective output clock signal with arespective frequency from each respective frac-N PLL based on theon-chip reference clock signal distributed (1608). The method mayoperate each respective transmitter and receiver of each SerDes lane atrespective transmit and receive data rates that may be based on therespective frequency and may be independent of data rates of otherSerDes lanes of the plurality of SerDes lanes (1610) and the methodthereafter ends (1612) in the example embodiment.

FIG. 16B is flow diagram 1650 of another example embodiment of a methodfor achieving SerDes lane datarate independence (1653). The method maybegin (1652) and obtain a baseline for frac-N PLL performance of afrac-N PLL employed in a SerDes lane, such as disclosed above withreference to FIG. 1. The method may check for whether SerDes lanedatarate independence is desired (1656). If not, the method thereafterends (1664) in the example embodiment. However, if SerDes lane datarateindependence is desired, the method may add a sigma-delta (i.e., ρΔ)modulator (1658), such as the sigma-delta modulator 760 or thesigma-delta modulator 1460, disclosed above with reference to FIG. 7 andFIG. 14, respectively.

The method may check whether a performance of the frac-N PLL iscomparable to that of an integer PLL that employs a same integer dividevalue as the frac-N PLL (1660). If the performance is comparable, themethod thereafter ends (1664) in the example embodiment. However, if theperformance is not comparable, the method may perform a co-optimizationmethod (1662) with respect to (i) an on-chip frequency reference value(F_(ref)) of a frequency of an on-chip reference clock signaldistributed to the frac-N PLL, such as the on-chip reference clocksignal 110 of FIG. 1, disclosed above, (ii) an order of the sigma-deltamodulator, and (iii) out-of-band poles. For example, the co-optimizationmethod (1662) may increase the on-chip frequency reference value,increase the order of the sigma-delta modulator, or add out of bandpole(s), or a combination thereof. Following the co-optimization method(1662), the method may again check whether the performance of the frac-NPLL is comparable to that of an integer PLL that employs a same integerdivide value as the frac-N PLL (1660).

If yes, the method thereafter ends (1664) in the example embodiment. Ifno, the method may again employ the optimization method (1662). As such,the method (1653) may iterate an optimization loop 1651 an iterateperforming the optimization method (1662) until the performance of thefrac-N PLL is comparable to that of an integer PLL that employs a sameinteger divide value as the frac-N PLL, enabling datarate independenceof the SerDes lane relative to other SerDes lanes. By recognizingtradeoffs between choosing an input frequency to a frac-N PLL, an orderof a sigma-delta modulator (i.e., ΣΔ), and out of band poles, aninfluence of quantization noise on the frac-N PLL performance may beminimized or eliminated while employing a frac-N PLL.

It should be understood that the method (1653) may be employed fromSerDes lane-to-lane or employed for a single lane and applied tomultiple SerDes lanes. For example, selection of input frequency to afrac-N PLL, an order of a sigma-delta modulator (i.e., ΣΔ), andout-of-band poles determined as a result of the optimization loop 1651,may be applied to all SerDes lanes of a circuit, such as the circuit 102of FIG. 1, disclosed above. Such selection may be based on all dataratesrequired to be supported by the circuit.

Alternatively, local frequency division of the on-chip frequencyreference value may be employed with any given SerDes lane and eachSerDes lane may employ a respective order of its respective sigma-deltamodulator and respective out-of-band poles that are selected for thegiven SerDes lane independent of selections made for other SerDes lanes.Factors such as power and performance requirements may influence theselection of the input frequency to the frac-N PLL, the order of asigma-delta modulator (i.e., ΣΔ), and the out-of-band poles that may beemployed.

While example embodiments have been particularly shown and described, itwill be understood by those skilled in the art that various changes inform and details may be made therein without departing from the scope ofthe embodiments encompassed by the appended claims.

What is claimed is:
 1. A circuit on a chip for serial data applications,the circuit comprising: a common phase-locked loop (PLL) having amultiplying factor, the common PLL configured to receive an off-chipreference clock signal generated external to the chip and produce anon-chip reference clock signal, the on-chip reference clock signalhigher in frequency relative to the off-chip reference clock signal; anda plurality of serializer/deserializer (SerDes) lanes each including arespective transmitter, a respective receiver, and a respectivefractional-N (frac-N) PLL including a respective divider with arespective divide value, the on-chip reference clock signal distributedto each respective frac-N PLL for use in generating a respective outputclock signal with a respective frequency, the respective transmitter andreceiver of each SerDes lane configured to operate at respectivetransmit and receive data rates that are based on the respectivefrequency and independent of data rates of other SerDes lanes of theplurality of SerDes lanes, the multiplying factor configured to suppressa portion of quantization noise, the portion introduced by modulatingthe respective divide value of the respective divider of each respectivefrac-N PLL of the plurality of SerDes lanes.
 2. The circuit of claim 1,wherein the off-chip reference clock signal is configured to operate ina low frequency range of 100 MHz to 156 MHz and the on-chip referenceclock signal is configured to operate in a higher frequency range of 400MHz to 600 MHz.
 3. The circuit of claim 1, wherein the circuit isconfigured to suppress a respective modulation noise, generated by eachrespective frac-N PLL of the plurality of SerDes lanes, by increasingfrequency of the respective modulation noise to enable each respectivefrac-N PLL to filter the respective modulation noise.
 4. The circuit ofclaim 1, wherein each respective frac-N PLL of the plurality of SerDeslanes further includes a respective sigma-delta modulator configured tomodulate, randomly, the respective divide value of the respectivedivider.
 5. The circuit of claim 1, wherein the common PLL is an integerPLL.
 6. The circuit of claim 1, wherein the portion is an initialportion and wherein each respective frac-N PLL of the plurality ofSerDes lanes further includes a respective out-of-band parasitic poleconfigured to suppress a remaining portion of the quantization noise,the remaining portion at higher offset frequencies relative to offsetfrequencies of the initial portion, the initial portion and theremaining portion suppressed below a noise level of noise generated fromintrinsic phase detector and voltage controlled oscillator (VCO) noisesources on the chip.
 7. The circuit of claim 1, wherein the common PLLis a first integer PLL, wherein each respective frac-N PLL of theplurality of SerDes lanes further includes a respective out-of-bandparasitic pole configured high enough to not affect closed-loop behaviorof the respective frac-N PLL, and wherein each respective out-of-bandparasitic pole of each respective frac-N PLL of the plurality of SerDeslanes in combination with configuration of the multiplying factor enablejitter and phase noise performance of each respective frac-N PLL of theplurality of SerDes lanes to be comparable to that of a second integerPLL that employs a same integer divide value as the respective frac-NPLL.
 8. The circuit of claim 1, wherein each respective frac-N PLL ofthe plurality of SerDes lanes further includes a respective sigma-deltamodulator and wherein each respective frac-N PLL of the plurality ofSerDes lanes has an order that is lower relative to that of therespective sigma-delta modulator.
 9. The circuit of claim 8, wherein theorder of each respective frac-N PLL of the plurality of SerDes lanes istwo and that of the respective sigma-delta modulator is three.
 10. Thecircuit of claim 1, wherein each respective frac-N PLL of the pluralityof SerDes lanes further includes a respective spread spectrum clock(SSC) pattern generator configured to modulate the respective frac-NPLL, directly, according to a respective SSC profile.
 11. The circuit ofclaim 1, wherein: the chip is a physical layer device (PHY); each SerDeslane is configured to employ the respective transmit and receive datarates to communicate with a respective media access controller (MAC);and the respective transmit and receive data rates are configuredindependent of data rates employed by other SerDes lanes of the PHY forcommunicating with other MACs.
 12. A method for achievingserializer/deserializer (SerDes) lane datarate independence, the methodcomprising: producing an on-chip reference clock signal from a commonphase-locked loop (PLL) having a multiplying factor, the common PLL on achip, the on-chip reference clock signal produced based on an off-chipreference clock signal generated external to the chip, the on-chipreference clock signal higher in frequency relative to the off-chipreference clock signal; and distributing the on-chip reference clocksignal to each respective fractional-N (frac-N) PLL of a plurality ofSerDes lanes each including a respective transmitter, a respectivereceiver, and a respective frac-N PLL including a respective dividerwith a respective divide value; modulating the respective divide valueof the respective divider of each respective frac-N PLL of the pluralityof SerDes lane, the multiplying factor configured to suppress a portionof quantization noise introduced by the modulating; generating arespective output clock signal with a respective frequency from eachrespective frac-N PLL based on the on-chip reference clock signaldistributed; and operating each respective transmitter and receiver ofeach SerDes lane at respective transmit and receive data rates that arebased on the respective frequency and independent of data rates of otherSerDes lanes of the plurality of SerDes lanes.
 13. The method of claim12, further comprising operating the off-chip reference clock signal ina low frequency range of 100 MHz to 156 MHz and operating the on-chipreference clock signal in a higher frequency range of 400 MHz to 600MHz.
 14. The method of claim 12, further comprising suppressing arespective modulation noise, generated by each respective frac-N PLL ofthe plurality of SerDes lanes, by increasing frequency of the respectivemodulation noise to enable each respective frac-N PLL to filter therespective modulation noise.
 15. The method of claim 12, wherein eachrespective frac-N PLL of the plurality of SerDes lanes further includesa respective sigma-delta modulator and wherein the modulating includesemploying each respective sigma-delta modulator to modulate, randomly,the respective divide value of the respective divider.
 16. The method ofclaim 12, wherein the common PLL is an integer PLL.
 17. The method ofclaim 12, wherein the portion is an initial portion, wherein eachrespective frac-N PLL of the plurality of SerDes lanes further includesa respective out-of-band parasitic pole, and wherein the method furthercomprises configuring each respective out-of-band parasitic pole tosuppress a remaining portion of the quantization noise, the remainingportion at higher offset frequencies relative to offset frequencies ofthe initial portion, the initial portion and the remaining portionsuppressed below a noise level of noise generated from intrinsic phasedetector and voltage controlled oscillator (VCO) noise sources on thechip.
 18. The method of claim 12, wherein the common PLL is a firstinteger PLL, wherein each respective frac-N PLL of the plurality ofSerDes lanes further includes a respective out-of-band parasitic pole,and wherein the method further comprises configuring each respectiveout-of-band parasitic pole of each respective frac-N PLL of theplurality of SerDes to be high enough to not affect closed-loop behaviorof the respective frac-N PLL and wherein each respective out-of-bandparasitic pole of each respective frac-N PLL of the plurality of SerDeslanes in combination with configuration of the multiplying factor enablejitter and phase noise performance of each respective frac-N PLL of theplurality of SerDes lanes to be comparable to that of a second integerPLL that employs a same integer divide value as the respective frac-NPLL.
 19. The method of claim 12, wherein each respective frac-N PLL ofthe plurality of SerDes lanes further includes a respective sigma-deltamodulator and wherein the method further comprises configuring eachrespective frac-N PLL of the plurality of SerDes lanes to have an orderthat is lower relative to that of the respective sigma-delta modulator.20. The method of claim 19, wherein the order of each respective frac-NPLL of the plurality of SerDes lanes is two and that of the respectivesigma-delta modulator is three.
 21. The method of claim 12, wherein eachrespective frac-N PLL of the plurality of SerDes lanes further includesa respective spread spectrum clock (SSC) pattern generator and themethod further comprises configuring each respective SSC patterngenerator to modulate the respective frac-N PLL, directly, according toa respective SSC profile.
 22. The method of claim 12, wherein the chipis a physical layer device (PHY), and the method further comprisesconfiguring each SerDes lane to employ respective transmit and receivedata rates to communicate with a respective media access controller(MAC) and configuring the respective transmit and receive data ratesindependent of data rates employed by other SerDes lanes of the PHY forcommunicating with other MACs.
 23. A circuit on a chip for serial dataapplications, the circuit comprising: a common phase-locked loop (PLL),the common PLL configured to receive an off-chip reference clock signalgenerated external to the chip and produce an on-chip reference clocksignal, the on-chip reference clock signal higher in frequency relativeto the off-chip reference clock signal; and a plurality ofserializer/deserializer (SerDes) lanes each including a respectivetransmitter, a respective receiver, and a respective fractional-N(frac-N) PLL including a respective sigma-delta modulator, the on-chipreference clock signal distributed to each respective frac-N PLL for usein generating a respective output clock signal with a respectivefrequency, the respective transmitter and receiver of each SerDes laneconfigured to operate at respective transmit and receive data rates thatare based on the respective frequency and independent of data rates ofother SerDes lanes of the plurality of SerDes lanes, wherein eachrespective frac-N PLL of the plurality of SerDes lanes has an order thatis lower relative to that of the respective sigma-delta modulator.
 24. Amethod for achieving serial/deserializer (SerDes) lane datarateindependence, the method comprising: producing an on-chip referenceclock signal from a common phase-locked loop (PLL) on a chip based on anoff-chip reference clock signal generated external to the chip, theon-chip reference clock signal higher in frequency relative to theoff-chip reference clock signal; distributing the on-chip referenceclock signal to each respective fractional-N (frac-N) PLL of a pluralityof SerDes lanes each including a respective transmitter, a respectivereceiver, and a respective frac-N PLL including a respective sigma-deltamodulator, each respective frac-N PLL of the plurality of SerDes lanesconfigured to have an order that is lower relative to that of therespective sigma-delta modulator; generating a respective output clocksignal with a respective frequency from each respective frac-N PLL basedon the on-chip reference clock signal distributed; and operating eachrespective transmitter and receiver of each SerDes lane at respectivetransmit and receive data rates that are based on the respectivefrequency and independent of data rates of other SerDes lanes of theplurality of SerDes lanes.
 25. A circuit on a chip for serial dataapplications, the circuit comprising: a common phase-locked loop (PLL)having a multiplying factor, the common PLL configured to receive anoff-chip reference clock signal generated external to the chip and toproduce an on-chip reference clock signal, the on-chip reference clocksignal higher in frequency relative to the off-chip reference clocksignal; and a serializer/deserializer (SerDes) lane, the SerDes laneincluding a fractional-N (frac-N) PLL, the frac-N PLL including adivider with a divide value, the on-chip reference clock signaldistributed to the frac-N PLL, the multiplying factor configured tosuppress a portion of quantization noise, the portion introduced bymodulating the divide value of the divider of the frac-N PLL.
 26. Amethod comprising: producing an on-chip reference clock signal from acommon phase-locked loop (PLL) having a multiplying factor, the commonPLL on a chip, the on-chip reference clock signal produced based on anoff-chip reference clock signal generated external to the chip, theon-chip reference clock signal higher in frequency relative to theoff-chip reference clock signal; distributing the on-chip referenceclock signal to a fractional-N (frac-N) PLL of a serializer/deserializer(SerDes) lane, the frac-N PLL including a respective divider with arespective divide value; modulating the divide value of the divider ofthe frac-N PLL of the SerDes lane, the multiplying factor configured tosuppress a portion of quantization noise introduced by the modulating.27. A circuit on a chip for serial data applications, the circuitcomprising: a common phase-locked loop (PLL), the common PLL configuredto receive an off-chip reference clock signal generated external to thechip and to produce an on-chip reference clock signal, the on-chipreference clock signal higher in frequency relative to the off-chipreference clock signal; and a serializer/deserializer (SerDes) laneincluding a fractional-N (frac-N) PLL, the frac-N PLL including asigma-delta modulator, the on-chip reference clock signal distributed tothe frac-N PLL, the frac-N PLL configured to have an order that is lowerrelative to that of the sigma-delta modulator of the frac-N PLL of theSerDes lane.
 28. A method comprising: producing an on-chip referenceclock signal from a common phase-locked loop (PLL) on a chip based on anoff-chip reference clock signal generated external to the chip, theon-chip reference clock signal higher in frequency relative to theoff-chip reference clock signal; and distributing the on-chip referenceclock signal to a fractional-N (frac-N) PLL of a serializer/deserializer(SerDes) lane, the frac-N PLL including a sigma-delta modulator, thefrac-N PLL configured to have an order that is lower relative to that ofthe sigma-delta modulator of the frac-N PLL of the SerDes lane.